configure google zoom translation to return null if null passed in

git-svn-id: http://svn.openlayers.org/trunk/openlayers@920 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/Google.js

@@ -356,20 +356,30 @@ OpenLayers.Layer.Google.prototype = Object.extend( new OpenLayers.Layer(), {
      * @param {int} gZoom
      * 
      * @returns An OpenLayers Zoom level, translated from the passed in gZoom
+     *          Returns null if null value is passed in
      * @type int
      */
     getOLZoomFromGZoom: function(gZoom) {
-        return (gZoom - 1);
+        var zoom = null;
+        if (gZoom != null) {
+            zoom = gZoom - 1;
+        }
+        return zoom;
     },
     
     /**
      * @param {int} olZoom
      * 
      * @returns A GZoom level, translated from the passed in olZoom
+     *          Returns null if null value is passed in
      * @type int
      */
     getGZoomFromOLZoom: function(olZoom) {
-        return (olZoom + 1);
+        var zoom = null; 
+        if (olZoom != null) {
+            zoom = olZoom + 1;
+        }
+        return zoom;
     },
 
   //